Set time to 00 00 00

Question

I have a problem resetting hours in Java  For a given date I want to set the hours to 00 00 00   This is my code                Resets milliseconds  seconds  minutes and hours from the provided date                param date         return             public static Date trim Date date            Calendar calendar   Calendar getInstance            calendar setTime date           calendar set Calendar MILLISECOND  0           calendar set Calendar SECOND  0           calendar set Calendar MINUTE  0           calendar set Calendar HOUR  0            return calendar getTime            The problem is that sometimes the time is 12 00 00 and sometimes it is 00 00 00 and when I query the database for an entity that was saved on 07 02 2013 00 00 00 and the actual entity time  that is stored  is 12 00 00 the query fails    I know that 12 00 00    00 00 00    I am using AppEngine  Is this an appengine bug  problem or some other issue   Or does it depend on something else

User · Answer

tl dr  myJavaUtilDate                                    The terrible  java util Date  class is now legacy  Use  java time  instead   toInstant                                        Convert this moment in UTC from the legacy class  Date  to the modern class  Instant    atZone  ZoneId of   Africa Tunis                 Adjust from UTC to the wall-clock time used by the people of a particular region  a time zone    toLocalDate                                      Extract the date-only portion   atStartOfDay  ZoneId of   Africa Tunis           Determine the first moment of that date in that zone  The day does  not  always start at 00 00 00    java time  You are using terrible old date-time classes that were supplanted years ago by the modern java time classes defined in JSR 310   Date   Instant  A java util Date represent a moment in UTC  Its replacement is Instant  Call the new conversion methods added to the old classes   Instant instant   myJavaUtilDate toInstant       Time zone  Specify the time zone in which you want your new time-of-day to make sense   Specify a proper time zone name in the format of Continent Region  such as America Montreal  Africa Casablanca  or Pacific Auckland  Never use the 2-4 letter abbreviation such as EST or IST as they are not true time zones  not standardized  and not even unique       ZoneId z   ZoneId of   America Montreal        ZonedDateTime  Apply the ZoneId to the Instant to get a ZonedDateTime  Same moment  same point on the timeline  but different wall-clock time   ZonedDateTime zdt   instant atZone  z       Changing time-of-day  You asked to change the time-of-day  Apply a LocalTime to change all the time-of-day parts  hour  minute  second  fractional second  A new ZonedDateTime is instantiated  with values based on the original  The java time classes use this immutable objects pattern to provide thread-safety   LocalTime lt   LocalTime of  15   30         3 30 PM  ZonedDateTime zdtAtThreeThirty   zdt with  lt        First moment of day  But you asked specifically for 00 00  So apparently you want the first moment of the day  Beware  some days in some zones do not start at 00 00 00  They may start at another time such as 01 00 00 because of anomalies such as Daylight Saving Time  DST     Let java time determine the first moment  Extract the date-only portion  Then pass the time zone to get first moment   LocalDate ld   zdt toLocalDate     ZonedDateTime zdtFirstMomentOfDay   ld atStartOfDay  z       Adjust to UTC  If you need to go back to UTC  extract an Instant   Instant instant   zdtFirstMomentOfDay toInstant       Instant   Date  If you need a java util Date to interoperate with old code not yet updated to java time  convert   java util Date d   java util Date from  instant

User · Answer

Use another constant instead of Calendar HOUR  use Calendar HOUR OF DAY   calendar set Calendar HOUR OF DAY  0     Calendar HOUR uses 0-11  for use with AM PM   and Calendar HOUR OF DAY uses 0-23   To quote the Javadocs      public static final int HOUR      Field number for get and set indicating   the hour of the morning or afternoon  HOUR is used for the 12-hour   clock  0 - 11   Noon and midnight are represented by 0  not by 12    E g   at 10 04 15 250 PM the HOUR is 10    and     public static final int HOUR OF DAY      Field number for get and set   indicating the hour of the day  HOUR OF DAY is used for the 24-hour   clock  E g   at 10 04 15 250 PM the HOUR OF DAY is 22    Testing   now  is currently c  14 55 on July 23  2013 Pacific Daylight Time    public class Main      static SimpleDateFormat sdf   new SimpleDateFormat  yyyy-MM-dd HH mm ss         public static void main String   args                Calendar now   Calendar getInstance            now set Calendar HOUR  0           now set Calendar MINUTE  0           now set Calendar SECOND  0           System out println sdf format now getTime              now set Calendar HOUR OF DAY  0           System out println sdf format now getTime                Output     javac Main java   java Main 2013-07-23 12 00 00 2013-07-23 00 00 00

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As Java8 add new Date functions  we can do this easily           If you have instant  then      Instant instant1   Instant now        Instant day1   instant1 truncatedTo ChronoUnit DAYS       System out println day1     2019-01-14T00 00 00Z         If you have Date  then      Date date   new Date        Instant instant2   date toInstant        Instant day2   instant2 truncatedTo ChronoUnit DAYS       System out println day2     2019-01-14T00 00 00Z         If you have LocalDateTime  then      LocalDateTime dateTime   LocalDateTime now        LocalDateTime day3   dateTime truncatedTo ChronoUnit DAYS       System out println day3     2019-01-14T00 00     String format   day3 format DateTimeFormatter ISO LOCAL DATE TIME       System out println format    2019-01-14T00 00 00

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One more JAVA 8 way   LocalDateTime localDateTime   LocalDateTime now   truncatedTo ChronoUnit HOURS     But it s a lot more useful to edit the date that already exists

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Another simple way  final Calendar today   Calendar getInstance            today setTime new Date             today clear Calendar HOUR OF DAY           today clear Calendar HOUR           today clear Calendar MINUTE           today clear Calendar SECOND           today clear Calendar MILLISECOND

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You can either do this with the following   Calendar cal   Calendar getInstance    cal set year  month  dayOfMonth  0  0  0   Date date   cal getTime

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We can set java util Date time part to 00 00 00 By using LocalDate class of Java 8 Joda-datetime api   Date datewithTime   new Date        ex  Sat Apr 21 01 30 44 IST 2018 LocalDate localDate   LocalDate fromDateFields datewithTime   Date datewithoutTime   localDate toDate       Sat Apr 21 00 00 00 IST 2018

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Here are couple of utility functions I use to do just this          sets all the time related fields to ZERO         param date        return Date with hours  minutes  seconds and ms set to ZERO      public static Date zeroTime  final Date date         return DateTimeUtil setTime  date  0  0  0  0              Set the time of the given Date        param date     param hourOfDay     param minute     param second     param ms        return new instance of java util Date with the time set     public static Date setTime  final Date date  final int hourOfDay  final int minute  final int second  final int ms         final GregorianCalendar gc   new GregorianCalendar        gc setTime  date        gc set  Calendar HOUR OF DAY  hourOfDay        gc set  Calendar MINUTE  minute        gc set  Calendar SECOND  second        gc set  Calendar MILLISECOND  ms        return gc getTime

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You would better to primarily set time zone to the DateFormat component like this   DateFormat dateFormat   new SimpleDateFormat  HH mm ss    dateFormat setTimeZone TimeZone getTimeZone  GMT       Then you can get  00 00 00  time by passing 0 milliseconds to formatter   String time   dateFormat format 0     or you can create Date object   Date date   new Date 0      also pass milliseconds String time   dateFormat foramt date     or you be able to have more possibilities using Calendar component but you should also set timezone as GMT to calendar instance   Calendar calendar   Calendar getInstance TimeZone getTimeZone  GMT    Locale US   calendar set Calendar HOUR OF DAY  5   calendar set Calendar MINUTE  37   calendar set Calendar SECOND  27    dateFormat format calendar getTime

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Another way to do this would be to use a DateFormat without any seconds   public static Date trim Date date        DateFormat format   new SimpleDateFormat  dd MM yyyy        Date trimmed   null      try           trimmed   format parse format format date          catch  ParseException e        will never happen     return trimmed

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If you need format 00 00 00 in string  you should use SimpleDateFormat as below  Using  H  instead  h    Date today   new Date    SimpleDateFormat ft   new SimpleDateFormat  dd-MM-yyyy HH mm ss       not SimpleDateFormat  dd-MM-yyyy hh mm ss   Calendar calendarDM   Calendar getInstance    calendarDM setTime today   calendarDM set Calendar HOUR  0   calendarDM set Calendar MINUTE  0   calendarDM set Calendar SECOND  0   System out println  Current Date      ft format calendarDM getTime         Result is  Current Date  29-10-2018 00 00 00

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java time  Using the java time framework built into Java 8 and later  See Tutorial   import java time LocalTime  import java time LocalDateTime   LocalDateTime now   LocalDateTime now      2015-11-19T19 42 19 224   start of a day now with LocalTime MIN     2015-11-19T00 00 now with LocalTime MIDNIGHT     2015-11-19T00 00   If you do not need time-of-day  hour  minute  second etc  parts  consider using LocalDate class   LocalDate now      2015-11-19

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Doing this could be easier  In Java 8    LocalTime ofNanoOfDay 0

[java] Set time to 00:00:00

tl;dr

java.time

`Date` ? `Instant`

Time zone

`ZonedDateTime`

Changing time-of-day

First moment of day

Adjust to UTC

`Instant` ? `Date`

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